Process for removing natural gas liquids from a gaseous natural gas stream at elevated pressure to obtain a gaseous product stream having a reduced content of natural gas liquids is provided.
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1. A method of removing natural gas liquids from a gaseous natural gas stream at elevated pressure to produce a gaseous product stream having a reduced content of natural gas liquids, which method comprises the steps of:
(a) cooling the natural gas stream;
(b) introducing said cooled natural gas stream into the bottom of a scrub column that has a lower stripping section and an upper absorption section, each section containing at least one theoretical stage;
(c) allowing natural gas to rise through the scrub column, and removing from the top of the scrub column an overhead stream;
(d) partly condensing the overhead stream, and separating said partly condensed overhead stream into a gaseous stream having a reduced content of natural gas liquids and a liquid reflux stream, and removing the gaseous stream as the gaseous product stream having a reduced content of natural gas liquids;
(e) splitting the liquid reflux stream into a first reflux stream and a second reflux stream;
(f) introducing the first reflux stream into the top of the absorption section of the scrub column;
(g) introducing the second reflux stream into the top of the stripping section to strip the desired light gaseous components; and
(h) removing from the bottom of the scrub column a liquid bottom stream rich in heavier components.
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The present invention relates to removing natural gas liquids from a gaseous natural gas stream.
U.S. Pat. No. 5,325,673 publication describes a method of pre-treating a natural gas stream for liquefaction by removing heavier hydrocarbons from it, comprising the steps of:
The process is directed to obtaining an overhead product that has a very low concentration of C6+ hydrocarbons. However, the overhead product still contains considerable amounts of ethane, propane and butanes.
A method of removing natural gas liquids from a gaseous natural gas stream at elevated pressure to produce a gaseous product stream having a reduced content of natural gas liquids is provided, which method comprises the steps of:
In the specification and the claims the term natural gas liquids is used to refer to heavier hydrocarbons, such as liquefied petroleum gas components and natural gasolines.
The natural gas liquids are removed, because they have an economic value and because removing the natural gas liquids reduces the heating value of the natural gas stream.
By way of example we give a molar composition of a natural gas stream from which the natural gas liquids are to be removed: methane 86 mol %, ethane 6 mol %, propane 4 mol %, butane plus 1 mol %, and the balance is formed by other components, such nitrogen, carbon dioxide and helium.
The present invention relates in one aspect to removing such natural gas liquids from a natural gas stream at elevated pressure, for example at pressures greater than 3 MPa (absolute) and less than the critical pressure of the natural gas, which is about 7 MPa (absolute).
A method for removing natural gas liquids from a gaseous natural gas stream is provided wherein high-pressure separation is obtained, which has a high propane recovery and a high rejection of methane and ethane. Moreover the process provides a method of removing natural gas liquids in which no reboiler is needed.
To this end the method of removing natural gas liquids from a gaseous natural gas stream at elevated pressure to obtain a gaseous product stream having a reduced content of natural gas liquids compared to the gaseous natural gas stream is provided comprising the steps of:
Suitably the method according to the present invention further comprises introducing hydrocarbon liquid into the top of the absorption section.
The invention will now be described by way of example in more detail with reference to the accompanying drawings.
Process for removing natural gas liquids from a gaseous natural gas stream at elevated pressure to obtain a gaseous product stream having a reduced content of natural gas liquids is provided:
Reference is first made to
The scrub column 6 operates at the pressure at which the natural gas is supplied. The scrub column 6 comprises two sections, a lower stripping section 7 and an upper absorption section 8 separated from the stripping section 7 by an interval 9. The stripping section 7 comprises between 1 and 4 theoretical separation stages and the absorption section 8 comprises between 4 and 10 theoretical stages. The theoretical separation stages can be provided by contacting trays or by a suitable packing material.
The gaseous fraction of the natural gas stream is allowed to rise in the scrub column 6 through the stripping section 7 and the absorption section 8. The liquid fraction of the natural gas stream is removed from the scrub column 6 through a conduit 10.
From the top of the scrub column 6 an overhead stream is removed through conduit 12, which overhead stream has a reduced content of natural gas liquids. The overhead stream is partly condensed in heat exchanger 14, and separated in separation vessel 17 into a liquid stream and a gaseous product stream. The gaseous product stream is removed from the separation vessel 17 through conduit 20 and passed to a plant for liquefying the gaseous product stream (not shown). The liquid stream is removed through conduit 21. The temperature of the partly condensed overhead stream is in the range of from −25 to −65° C., and the amount of liquid in the partly condensed overhead stream is between 10 and 35 mol % based on the total overhead stream. Suitably the gaseous product stream removed through conduit 20 is supplied (not shown) to the heat exchanger 14 to provide the cold for partly condensing the overhead stream, before the gaseous product is transported away.
Part of the liquid stream is introduced through conduit 22 as a first reflux stream into the top of the scrub column 6 above the absorption section 8 as an absorbent. The liquid is brought in the absorption section 8 in counter-current contact with the gas from the stripping section 7. Components heavier than methane are removed from the gas by the first reflux stream acting as an absorbent.
The remainder of the liquid stream is introduced through conduit 23 as a second reflux stream into the scrub column 6 in the interval 9 above the stripping section 7. In the stripping section 7, the second reflux stream and the liquid descending from the absorption section 8 are brought into counter-current contact with the rising gaseous fraction of the natural gas stream. The gaseous fraction strips the light components (methane and ethane) from the liquid stream. The liquid stream having a low concentration of light components is then removed from the bottom of the scrub column 6 through conduit 10.
Suitably, the amount of second reflux stream in the conduit 23 is between 10 and 95% by mass of the liquid stream that is removed from the separator vessel 17.
In addition hydrocarbon liquid can be introduced into the top of the absorption section 8 through conduit 25. A suitable hydrocarbon liquid is butane. The amount of this additional absorbent is suitably between 1 and 4 times the amount of liquid supplied through conduit 22. The additional absorbent suitably consists of butane plus components.
Reference is now made to
From the bottom of the stripping column 30 a liquid stream is removed through conduit 35. Part of the liquid bottom stream is vaporised in reboiler 36 and the resulting vapour is introduced into the bottom of the stripping column 30. The remainder is passed away through conduit 38 to storage (not shown) or to a further treatment (not shown).
From the top of the stripping column 30 a gaseous overhead stream is removed through conduit 40. The gaseous overhead stream is partly condensed in heat exchanger 43 to obtain a partly condensed gaseous overhead stream. The partly condensed overhead stream is separated in separator 46 into a liquid fraction and a gaseous fraction. The liquid fraction is removed through conduit 48 and introduced into the top of the stripping column 30 as reflux. The gaseous fraction is removed through conduit 50 and added to the gaseous product stream. Optionally the pressure of the gaseous overhead fraction is increased to the pressure of the gaseous product stream using compressor 53.
Suitably at least part of the gaseous product stream removed through conduit 20 is supplied to heat exchanger 14 to provide the cold for partly condensing the overhead stream 12.
Suitably the gaseous overhead is partly condensed by indirect heat exchange with the gaseous product stream supplied to the heat exchanger 43 through conduit 20.
Suitably the liquid bottom stream removed from the scrub column 6 through conduit 10 is cooled by indirect exchange in heat exchanger 55 with the gaseous product stream.
In heat exchanger 2 the natural gas stream in conduit 1 is partly condensed, and suitably this is done by indirect heat exchange with the gaseous product stream.
Suitably the hydrocarbon liquid supplied to the top of the scrub column 6 is cooled in heat exchanger 57 by indirect heat exchange with the gaseous overhead fraction.
The invention will now be described by way of example with reference to three calculated examples. In the first example, not according to the present invention, only the reflux through conduit 22 is supplied to the top of the scrub column 6. In the second example, according to the present invention, reflux is supplied through conduits 22 and 23; and in the third example, according to the present invention, additional hydrocarbon liquid is supplied through conduit 25. The conditions for each example have been so selected as to maximize the recovery of natural gas liquids.
In the examples, the scrub column contains eight theoretical separation stages. In the two examples according to the present invention, the stripping section 7 contains two theoretical separation stages and the absorption section 8 contains six theoretical separation stages.
The results are summarized in the below Tables 1–8.
In the tables the molar flow rates are in kmol/s, the mass flow rates are in kg/s, the temperatures are in ° C., the pressures are in MPa (absolute), and the molar composition is in mol %. The butane plus components are butane, iso-butane, pentane, iso-pentane, hexane and heptane. The other components mentioned in the composition are water, nitrogen, hydrogen sulphide, carbon dioxide and helium.
TABLE 1
Data on partly condensed feed supplied
through conduit 3.
Example 1,
Not
Example 2,
Example 3,
according to
According to
According to
the
the
the
Stream 3
invention
invention
invention
Molar flow
7.90
7.55
7.73
rate
Mass flow
155
142
145
rate
Temperature
0
−5
−5
Pressure
5.6
5.8
6.0
Molar
composition
Methane
0.862
0.882
0.882
Ethane
0.064
0.062
0.062
Propane
0.042
0.036
0.036
Butane plus
0.031
0.019
0.019
Other
Balance
Balance
Balance
components
TABLE 2
Data on overhead stream removed from top of
the scrub column through conduit 12. Please note that
the flow rates are larger than the flow rates in
conduit 3, because of the internal recycle.
Example 1,
Not
Example 2,
Example 3,
according to
According to
According to
the
the
the
Stream 12
invention
invention
invention
Molar flow
8.79
8.64
8.39
rate
Mass flow
168
161
152
rate
Temperature
−19
−23
−18
Pressure
5.6
5.6
6.0
Molar
composition
Methane
0.852
0.871
0.897
Ethane
0.080
0.078
0.070
Propane
0.064
0.043
0.022
Butane plus
0.003
0.006
0.001
Other
Balance
Balance
Balance
components
TABLE 3
Data on reflux supplied through conduit 22
to top of the scrub column 6.
Example 1,
Not
Example 2,
Example 3,
according to
According to
According to
the
the
the
Stream 22
invention
invention
invention
Molar flow
1.57
0.17
0.08
rate
Mass flow
40.5
4.05
1.81
rate
Temperature
−43
−48
−54
Pressure
5.6
5.8
6.0
Molar
composition
Methane
0.571
0.650
0.720
Ethane
0.176
0.170
0.155
Propane
0.241
0.152
0.078
Butane plus
0.001
0.027
0.005
Other
Balance
Balance
Balance
components
TABLE 4
Data on reflux supplied through conduit 23
to top of the stripping section 7 in the scrub column 6.
Example 1,
Not
Example 2,
Example 3,
according to
According to
According to
the
the
the
Stream 23
invention
invention
invention
Molar flow
Not
1.54
1.46
rate
applicable
Mass flow
Not
3.68
3.31
rate
applicable
Temperature
Not
−48
−54
applicable
Pressure
Not
5.8
6.0
applicable
Molar
composition
Methane
Not
0.650
0.720
applicable
Ethane
Not
0.170
0.155
applicable
Propane
Not
0.152
0.078
applicable
Butane plus
Not
0.027
0.005
applicable
Other
Not
Balance
Balance
components
applicable
TABLE 5
Data on hydrocarbon liquid supplied through
conduit 25 to top of the scrub column 6.
Example 1,
Not
Example 2,
Example 3,
according to
According to
According to
the
the
the
Stream 25
invention
invention
invention
Molar flow
Not
Not
0.12
rate
applicable
applicable
Mass flow
Not
Not
7.9
rate
applicable
applicable
Temperature
Not
Not
−33
applicable
applicable
Pressure
Not
Not
6.0
applicable
applicable
Molar
composition
Methane
Not
Not
0.00
applicable
applicable
Ethane
Not
Not
0.00
applicable
applicable
Propane
Not
Not
0.00
applicable
applicable
Butane plus
Not
Not
1.00
applicable
applicable
Other
Not
Not
0.00
components
applicable
applicable
TABLE 6
Data on the gaseous product stream removed
through conduit 20.
Example 1,
Not
Example 2,
Example 3,
according to
According to
According to
the
the
the
Stream 20
invention
invention
invention
Molar flow
7.196
6.927
6.852
rate
Mass flow
128
120
109
rate
Temperature
−43
−49
−54
Pressure
5.3
5.5
5.7
Molar
composition
Methane
0.913
0.925
0.937
Ethane
0.059
0.055
0.051
Propane
0.026
0.017
0.009
Butane plus
0.0009
0.0011
0.0015
Other
Balance
Balance
Balance
components
TABLE 7
Data on liquid fraction of the feed gas that
is removed from the bottom of the scrub column 6
through conduit 10.
Example 1,
Not
Example 2,
Example 3,
according to
According to
According to
the
the
the
Stream 10
invention
invention
invention
Molar flow
0.70
0.63
0.99
rate
Mass flow
27
22
36
rate
Temperature
−2
−11
−10
Pressure
5.6
5.8
6.0
Molar
composition
Methane
0.343
0.401
0.401
Ethane
0.108
0.133
0.133
Propane
0.208
0.255
0.218
Butane plus
0.341
0.210
0.247
Other
Balance
Balance
Balance
components
The results can be summarized in Table 8 by comparing the content of hydrocarbons in the liquid stream that is removed from the bottom of the scrub column 6 through conduit 10.
TABLE 8
Composition of the liquid stream removed
through conduit 10, in percent of the composition of
the feed supplied through conduit 1 (taking into
account the absorption liquid supplied through
conduit 25 in the last example).
Example 1,
Not
Example 2,
Example 3,
according to
According to
According to
the
the
the
Stream 10
invention
invention
invention
Methane
3.5
3.8
5.8
Ethane
15
18
27
Propane
44
59
78
Butane plus
97
93
94
The above results illustrate the advantageous effect on the recovery of natural gas liquids of separating the reflux stream into two reflux streams, a first reflux stream being supplied to the top of the scrub column 6 and a second reflux stream being supplied to the top of the stripping section 7.
The method according to the present invention can be used to remove heavier components from natural gas that is after processing introduced into a pipeline through which the gas is transported to the user.
However, suitably the method according to the present invention is used to remove heavier components from natural gas that is after processing supplied to a plant for liquefying natural gas.
Bras, Eduard Coenraad, Kumar, Paramasivam Senthil
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